Microelectronic devices such as semiconductor integrated circuits (“ICs”) are tested typically at full performance using complex and expensive IC testers. The temperature of the IC must be maintained at a fixed value during a test if test results are to be meaningful. In particular, significant deviations from the fixed value during the test may result in test failure of nominally good ICs, or in passing defective ICs as good units. However, maintaining the temperature of the IC at a fixed value becomes increasingly difficult as power dissipated by the IC is increased.
During testing of high power ICs, for example, the temperature of the high power IC is typically maintained at a fixed value by use of a thermal management plate it contact with a case of the IC. As such, the IC is maintained at a relatively stable temperature, notwithstanding variations in power dissipated by the IC during the test. As advances are made in the power and performance of modern ICs, accurate maintenance of temperature during testing is increasingly important in the manufacture of modern ICs and microelectronic devices.
Demands for increased reliability of microelectronic devices have led to testing these devices at two or more temperatures to detect defects that might not appear in testing performed at only one temperature. A typical approach to such testing involves, for example, testing at: (a) a low temperature of −55° C.; (b) a nominal operating temperature of 35° C.; and (c) a high temperature of +150°. In accordance with this approach, defects that appear only at a low temperature or at a high temperature may be detected. While it is efficient to mount the device on a socket once to do testing at each of the three temperatures, efficiency is lost because the socket and associated test electronics is idle while the temperature of the device is changed from one set point temperature to a next set point temperature. In response to this problem, several approaches have been tried in the prior art in attempts to reduce the time needed to make a transition from one set point temperature to the next, and thereby, to reduce idle time between tests done at these set point temperatures.
One prior art approach to setting and controlling temperature of a device under test involves flowing cooling fluid continuously through a thermal head (the test head is disposed in good thermal contact with the device) while the temperature of the cooling fluid is regulated by adding heat to, or removing heat from, the fluid—means for adding or removing heat include resistive heaters, heat exchangers, thermoelectric devices and refrigeration units. In practice, the device is kept at a first set point temperature by circulating the cooling fluid through the thermal head. Next, heat is added to the thermal head by, for example, resistive heating of an element in the thermal head to establish a second set point temperature. Finally, more heat is added to the thermal head, again by resistive heating, to establish a third set point temperature. This prior art approach enables additional heat to be added to or removed from the thermal head to compensate for changes in power dissipated by the device under test. Many variants of this approach have been tried in an effort to increase responsiveness in rapidly changing temperature without significantly degrading conduction of heat from the device to the cooling fluid. However, in most variants, one problem is that an increase in responsiveness is accompanied by a reduction in thermal conductivity of the thermal head. An additional problem results from a non-uniform temperature on the surface of the thermal head due to local resistive heating elements in the head. As such, this prior art approach of adding or removing heat is inadequate for responding to changes in a desired temperature rapidly.
Another prior art approach to setting and controlling temperature of a device under test involves using a Peltier effect cooler in a thermal head that is in contact on a first surface with the device under test and on a second surface with cooling fluid. In accordance with this approach, an electrical current flowing through junctions in the Peltier effect cooler induces heat transfer from the first surface to the second surface, thereby raising or lowering the temperature of the device under test in contact with the first surface with respect to a temperature of the cooling fluid. By controlling the magnitude and direction of the current, the temperature of the device is set and maintained. While Peltier effect coolers can be used to control temperature of a thermal head, their use is problematic in that their efficiency is inadequate for cooling high power devices or for controlling temperature over a wide range.
Yet another prior art approach to setting and controlling temperature of a device under test involves using several cooling fluids at two or more temperatures to set and control temperature of a thermal head. In accordance with this prior art approach, the fluids are mixed in a known proportion to establish a temperature of a fluid flowing through the thermal head. The temperature of the thermal head is changed by changing the proportion of fluids in confluence through the thermal head. This prior art approach is problematic in that control of temperature by mixing fluids of different temperatures is relatively slow compared to temperature changes induced by changes in instantaneous power dissipated by a device under test.
Yet another prior art approach to setting and controlling temperature of a device under test involves flowing a cooling fluid at a first temperature through an intake to a thermal head while a second cooling fluid at a second temperature is metered into the intake flow. By adjusting the metering rate, the temperature of the thermal head is controlled within limits set by the first and the second temperatures. Fluid exhausted from the thermal head is collected in a common return at a temperature intermediate between the first and second temperatures. This prior art approach suffers from a problem in that the exhaust flow of fluid must be brought to either the first or the second temperature before it can be circulated back to the thermal head, thereby resulting in delays and inefficiencies. Further, the fluid in the intake lines will change temperature due to losses to the ambient if a flow of each fluid is not maintained continuously.
Yet another prior art approach to setting and controlling temperature of a device under test involves regulating the temperature of a thermal head by turning on and off a flow of two or more thermal transfer fluids that flow through separate channels in a thermal head. Each of the fluids is obtained from a source at a different temperature. This prior art approach suffers from a problem in that a first fluid at a first temperature remains in the thermal test head after flow of the first fluid is shut off. This fluid must be heated by a flow of fluid at a second temperature, thereby slowing the transition of the thermal head from the first temperature to the second temperature. In addition, the thermal efficiency of such a thermal head is diminished because two or more fluid channels must be formed in the same thermal head, adding thermal mass that further slows the thermal response of the thermal head. This prior art approach also suffers from a problem in that there will be a non-uniformity in temperature due to the use of multiple channels in the thermal head that necessitates compromises between thermal efficiency and temperature uniformity.
Yet another prior art approach to setting and controlling temperature of a device under test involves regulating the temperature of a thermal head by turning on and off the flow of a multiplicity of thermal transfer fluids flowing through a channel in the thermal head. Each of the multiplicity of fluids is supplied at a different temperature, thereby allowing the temperature of the thermal head to be changed by switching on the flow of fluid of the desired temperature and switching off the flow of all other fluids. This prior art approach is problematic because it enables fluids in circuits that are switched off to change temperature by gain or loss of heat from the ambient while waiting to be switched on. Further, this prior art approach does not return fluid exhausted from the thermal head to the reservoir from whence it came. As such, the exhausted fluid must be heated or cooled before it is returned to the thermal head, causing additional inefficiencies and delays.
In light of the above, there is a need in the art for apparatus that solves one or more of the above-identified problems.